BREAKDOWN: Soybean cyst nematode is no longer being controlled by resistant varieties that have the same genetic resistance. The situation is similar to weeds developing resistance to herbicides that are used continuously.

SCN-resistant soybeans becoming less effective

Management of soybean cyst nematode relies heavily on the use of SCN-resistant soybean varieties to limit nematode reproduction and minimize yield loss. Greg Tylka, Iowa State University nematologist, has found that SCN-resistant soybean varieties are becoming less effective.

“This is an alarming trend and sets the stage for even greater yield loss from SCN in the future,” says Tylka.

Resistant varieties have helped farmers manage the nematodes for decades. Almost all SCN-resistant soybean varieties possess the same resistance genes, from a soybean breeding line called PI88788.

“In the 1990s, SCN was well controlled with PI88788 resistance. Starting in 2001, we saw a steady decrease in SCN control of varieties with PI88788 resistance,” says Tylka.

ROOT OF PROBLEM: It’s no longer uncommon to see SCN females on roots of SCN-resistant soybean varieties. There are more SCN-resistant soybean varieties than ever, but the diversity of that resistance is lacking.

The analysis indicates increasing damage from SCN to resistant soybean varieties. In some instances, resistant varieties allowed as much SCN feeding as susceptible varieties with no resistance. This trend will result in an increase of SCN numbers in the soil, which means lower yields.

The findings are similar to current trends in weeds that have developed resistance to herbicides. “The buildup of SCN on resistant soybean varieties is much like weeds developing resistance to glyphosate due to prolonged use of that single herbicide active ingredient,” notes Tylka.

SCN populations will continue to increase Tylka and his co-authors have concluded that SCN numbers will continue to increase and soybean yields will continue to decrease if PI88788 is the only SCN resistance farmers have available to use. One solution is to develop soybean varieties with resistance genes from breeding lines other than PI88788.

“This is a serious situation,” Tylka emphasizes, “because SCN has infested almost 70% of Iowa’s fields.”

The ISU research report concludes that long-term management of SCN requires a multifaceted and integrated approach. ISU is continuing to study use of resistant soybean varieties, seed treatments and whether cover crops can reduce SCN numbers.

If you are interested in learning more about the breakdown of SCN resistance and insecticide-resistant soybean aphids, plan to attend an all-day workshop at ISU’s Field Extension Education Lab, just west of Ames, on Aug. 17.